Re: [PATCH] staging: comedi: comedi_isadma: Use a non-NULL device for DMA API

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Apr 25, 2019 at 05:26:44PM +0100, Ian Abbott wrote:
> The "comedi_isadma" module calls `dma_alloc_coherent()` and
> `dma_free_coherent()` with a NULL device pointer which is no longer
> allowed.  If the `hw_dev` member of the `struct comedi_device` has been
> set to a valid device, that can be used instead.  Unfortunately, all the
> current users of the "comedi_isadma" module leave the `hw_dev` member
> set to NULL.  In that case, use a static dummy fallback device structure
> with the coherent DMA mask set to the ISA bus limit of 16MB.
> 
> Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
> ---
>  drivers/staging/comedi/drivers/comedi_isadma.c | 15 +++++++++++++--
>  drivers/staging/comedi/drivers/comedi_isadma.h |  3 +++
>  2 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/comedi/drivers/comedi_isadma.c b/drivers/staging/comedi/drivers/comedi_isadma.c
> index b77dc8d5d3ff..8929952516a1 100644
> --- a/drivers/staging/comedi/drivers/comedi_isadma.c
> +++ b/drivers/staging/comedi/drivers/comedi_isadma.c
> @@ -14,6 +14,16 @@
>  
>  #include "comedi_isadma.h"
>  
> +/*
> + * Fallback device used when hardware device is NULL.
> + * This can be removed after drivers have been converted to use isa_driver.
> + */
> +static struct device fallback_dev = {
> +	.init_name = "comedi_isadma fallback device",
> +	.coherent_dma_mask = DMA_BIT_MASK(24),
> +	.dma_mask = &fallback_dev.coherent_dma_mask,
> +};

Ick, no, static struct device are a very bad idea as this is a reference
counted structure and making it static can cause odd problems.

Why not just create a "real" one?  Or better yet, use the real device
for the comedi device as all of these drivers should have one now.

thanks,

greg k-h